Diffuser

ABSTRACT

A DIFFUSER ANNULUS INCLUDES A PLURALITY OF PASSAGES THERETHROUGH, WHEREIN THE PASSAGES GRADUALLY MERGE RADIALLY OUTWARD FROM A CURVILINEAR CROSS-SECTION TO A RECTILINEAR CROSS-SECTION. THE DIFFUSER ANNULUS MAY BE OPTIMIZED FOR MAXIMUM EFFICIENCY BY VARYING THE AXIALLY EXTENDING WIDTH OF THE PASSAGES AT THE OUTER PERIPHERY OF THE ANNULUS INDEPENDENTLY OF THE DIAMETER OF THE PASSAGES AT THE INNER PERIPHERY OF THE ANNULUS.   D R A W I N G

March 6, 1973 L t w. BLAIR Er AL DIFFUSER 2 Sheets-Sheet 2 Filed Aug. 24, 1971 e W Mw M www? wget M W m 6 y In f ,W 25 w United States Patent O 3,719,430 DIFFUSER Lawrence William Blair, Boxford, and Alexander Connor Bryans, Reading, Mass., assignors to General Electric Company Filed Aug. 24, 1971, Ser. No. 174,335 Int. Cl. Flild 1/02; F03d 1/04; F04d 29/40 U.S. Cl. 415-207 5 Claims ABSTRACT OF THE DISCLOSURE A diffuser annulus includes a plurality of passages therethrough, wherein the passages gradually merge radially outward from a curvilinear cross-section to a rectilinear cross-section. T he diffuser annulus may be optimized for maximum efficiency by varying the axially extending width of the passages at the outer periphery of the annulus independently of the diameter of the passages at the inner periphery of the annulus.

The invention described herein was made in the course of or under a contract or subcontract thereunder (or grant) with the Department of the Air Force.

BACKGROUND OF THE INVENTION This invention relates to a diffuser and more particularly to a centrifugal compressor diffuser which may be optimized for maximum efficiency regardless of the number and length of passages therethrough.

In most centrifugal compressors, a rotating impeller is arranged to add energy by accelerating the flow of gas therethrough. The centrifugal compressor diffuser is ordinarily characterized by an annular space surrounding the impeller in which the velocity of the fluid leaving the impeller is decreased and its static pressure is correspondingly increased. Circumferentially spaced apart guide vanes have generally been provided in most conventional diffusers to produce passages expanding in area for the purpose of diffusing the flow.

Diffusers having circumferentially spaced apart passages which are curvilinear in cross-section are well known to the art. Diffusers of this type are of particular advantage for use with todays gas turbine engines which are designed to accommodate extremely high flow velocities. The advantage to such diffusers lies in the reduction of flow separation and boundary layers which are precipitated within the corners of rectangular passages. However, flow exiting past a series of blunt edges as found between curvilinear passages becomes turbulent and incurs substantial flow losses. For smooth and uniform exit flow, thin edged vanes as found between rectilinear passages are most desirable. Therefore, as a compromise, it has been found preferable to have the passages initially assume a circular or curvilinear inlet cross-section and gradually merge into a rectangular outlet cross-section thereby minimizing iiow separation while still providing smooth and uniform exit flows from the diffuser.

Often in order to maximize the efficiency of conversion of kinetic energy into static pressure energy for a diffuser, it becomes necessary to vary the ratio of passage outlet area to inlet area while maintaining the passage length constant. However, the designer is limited by the following constraints. The length of the passages determines the radius of the annulus, and hence the outer circumference of the annulus. The total number of passages may also be predetermined by other design criteria so that when the outer circumference of the annulus is divided by the total number of passages, the circumferential dimension of the rectangular outlet cross-section becomes fixed and can not be varied. Previously, the other axially extending dimension of the rectangular passage outlet could not be made smaller than the diameter of the circular inlet cross-section without incurring a complicated and expensive machining operation or the addition of bulky extra hardware. Therefore, the designer was not always able to adjust the passage outlet area to inlet area ratio so as to achieve the maximum efficiency of conversion of kinetic energy to static pressure energy.

Therefore, it is an object of this invention to provide a diffuser wherein the efficiency may be optimized independently of the diffuser radius and the number of passages.

It is also an object of this invention to provide a diffuser wherein the area of the rectangular outlet cross-section of each passage may be varied independently of the circular inlet cross-section diameter of the passage by conventional manufacturing techniques Without resorting to complex machining operations or the addition of bulky extra hardware.

SUMMARY OF THE INVENTION defines one nominally linear edge of the rectilinear outlet cross-section.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood upon reading the following description of the preferred embodiment in conjunction with the accompanying drawings.

FIG. 1 shows a partial cross-sectional view of a conventional diffuser in combination with a centrifugal compressor.

FIG. 1(a) shows a cross-sectional view taken along the line lar-1a of FIG. 1.

FIG. 2 shows a graph divided into various areas of diffuser efficiency, and including a curvic representation for one diffuser having a predetermined number of passages.

IFIG. 3 shows a partially sectioned side View of the diffuser of this invention in combination with a centrifugal compressor.

FIG. 3(a) shows a cross-sectional view taken along the line 3a-3a of FIG. 3.

FIG. 4 shows a partially cut away perspective view of the diffuser of FIG. 3.

FIG. 5 shows a partial cross-sectional view taken along the central axis of the diffuser of FIGS. 3 and 4 in combination with a compressor and gas turbine engine.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 1(11) there is shown a portion of a centrifugal compressor wherein an impeller 1l)` is mounted for axial rotation about a center shaft 12. The impeller 10 includes a plurality of blades 14 circumferentially supported in spaced relationship by a web 16.

There is further shown a conventional annular diffuser 11 mounted externally so as to surround the outer tips of the blades 14. The conventional diffuser includes inner and outer peripheral surfaces 20 and 22, the inner Surface being closely adjacent to the outer -periphery of the impeller shown. The conventional diffuser includes a plurality of ciricumferentially spaced apart passages 24 extending through the annulus wherein the cross-sectional areas of the passages gradually widen in an outward radial direction. The passages convert the high kinetic energy with which the flowing medium leaves the impeller into static pressure energy exiting from the outer peripheral surface 22.

It is well known in the art to have the passages initially assume circular inlet cross-sections 25 and to gradually merge the passages into rectangular outlet cross-sections 26 so as to minimize flow separation and still provide a smooth and uniform exit ow from the diffuser as previously discussed. The inlet cross-sectional areas (Ai) of the passages 24 may be determined from the following equation:

where r1 is the radius of the circular cross-section 25. The outlet cross-sectional area (Ao) may be determined from the following equation:

Referring to FIG. 2, there is shown a graph having a plurality of curvilinear plots thereon, wherein the areas between each plot are representative of a level of efciency for conversion of kinetic energy into static pressure. The ordinate of the graph is a logarithmic scale calibrated to display varying ratios of outlet cross-sectional areas (A) to inlet cross-sectional areas (Ai) and the abscissa is a logarithmic scale calibrated to display varying ratios ofppassage lengths (L) to inlet cross-sectional radii (r1). The phantom curve A represents a characteristic plot for a difuser wherein the number of passages (Np) have been predetermined and remain constant and the axially extending width (W) has been minimized so as to equal 2r1. As may be readily observed, the curve A does not intersect the area of maximum efficiency, (80%) and therefore ditusers having this particular number of passages cannot be optimized for peak efficiency.

The number of diffuser passages may be changed so as to shift the curve A into intersection with the enclosed area of maximum efficiency; however, varying the number of diiuser passages to achieve optimum eiiiciency may not always be possible due to other design criteria.

Another parameter that may be varied to shift the location of the curve A is the axially extending width (W) of each passage. Referring back to the equation for the ratio of the outlet cross-sectional area t0 the inlet cross-sectional area (Ao)/(A), it can be seen that the width (W) may be decreased in order to decrease the ratio (A0)/ (Ai) for a particular ratio of passage length (L) to inlet cross-sectional radius (r1). The curve A could then be shifted toward the absicssa and away from the ordinate so as to intersect the area of maximum eciency. However, as previously mentioned, the width (W) has already been minimized to equal twice the radius (r1) of the circular inlet cross-sectional area in the conventional diiuser. The diffuser of this invention, however, allows a decrease in the width (W) to less than twice the radius (r1) thereby permitting etiiciency optimization for any diffuser regardless of the number of diffuser passages.

Referring now to FIGS. 3 and 3(a) where the numerals refer to previously described elements, there is shown a portion of a conventional centrifugal compressor in cornbination with the annular diffuser 30 of this invention. The annular diffuser 30, a portion of which is also shown in FIG. 4, is mounted externally so as to surround the outer tips of the blades 14. The annular diffuser includes inner and outer peripheral surfaces 31 and 32, the inner surface being closely adjacent to the outer periphery of the impeller as shown.

The diffuser 30 includes a plurality of ciricumferentially spaced apart passages 34 extending through the annulus so as to provide flow connection from the inner peripheral surface 31 to the outer peripheral surface 32. The cross-sectional areas of the passages gradually widen in an outward radial direction so as to convert the high kinetic energy with which the flowing medium leaves the impeller into static pressure energy. The passages 34 have been shown as initially assuming circular inlet cross-Sections 36 on the inner peripheral surface 31 and gradually merging into rectangular outlet cross-sections 3S on the outer peripheral surface 32.

Having the passages 34 initially assume a circular inlet cross-section, as previously discussed, reduces the boundary layer separation characteristic of square cornered rectangular forms which cause detachment of flow together with loss of eiciency. However, the passages 34 must also merge into rectangular cross-sections so that the uid exits past narrow, axially extending, trailing edges 40 and commingles in a smooth and uniform ow without the ow separation and losses associated with dumping a uid past a series of blunt edges.

In order to minimize the flow losses associated with rectangular forms, it has been found preferable not only to have the passages initially assume a circular inlet crosssection, but to also maintain the circular cross-section for the greater length of the passage and to confine the merging portion of the passage to the outmost radial end. The limiting factor as to how small the length of the merging portion of the passage may be in relation to the overall passage length is the angle of divergence of the passage walls from the centerline of the passage. As the length of the merging portion of the passage is decreased, the angle of divergence of the passage walls from the centerline increases until a point is reached where How separation from the walls becomes significant.

For the forementioned reasons, it has been found preferable to divide the passages 34 of the diffuser 30 into what are generally conical passages 44 between the circular inlet cross-sections 36 and intermediate circular cross-sections indicated by phantom lines 42 and what are generally merging passages 46 between the intermediate cross-sections 42 and the rectangular outlet crosssections 38 on the outer periphery 32. The merging passages 46 are initially circular in cross-section and gradually merge the ow into a rectangular cross-section 38. In order to minimize the length of the merging passage 46, it has been found preferable to maintain a uniform crosssectional area throughout the effective length of the merging passage so as to confine all substantial conversion of the flow from kinetic energy to static pressure within the conical passage 44.

While conical passages 44 have been shown as preferred, it is within the intended scope of invention that the passages may alternatively be any other formation that is curvilinear in cross-section. Also, although centerlines 47 of the passages 34 are shown as substantially straight, they may also be curved, and although the centerlines of the passages are shown as being in a transverse radial plane, they may also be inclined at a moderate angle to this plane without departing from the scope of the invention.

The outside circumferential edges of the diffuser 30 have been bevelled so as to provide two converging, scalloped, frusto-conical surfaces 48, 48', which are best shown in FIG. 4. The scalloped frusto-conical surfaces 48, 48 are contiguously engaged by annular frusto-conical discs 50, 50' respectively which converge in a generally outward radial direction. As can be readily observed from FIGS. 3 and 4, the outer radial edges 52, 52' of the annular frusto-conical discs 50, 50' cooperate with the axially extending trailing edges 40 to define the rectangular outlet cross-sections 38 on the outer periphery 32 of the annulus. Also, the inward radial penetration for the scalloped, frusto-conical surfaces 48, 48 preferably should not extend beyond the length of the merging passages 46. The discs 50, 50 may be attached to the scalloped, frusta-conical surfaces 48, 48' by conventional means such as brazing. Also of particular advantage is the fact that the passages may be machined by conventionl techniques such as milling or electrochemical machining.

It now becomes obvious that the unique diffuser of this invention does not suffer from the inherent design limitations of prior art diffusers. The axially extending width (W) of each rectangular outlet cross-section may be made smaller than the circular cross-section diameter 2r1. Referring back to the equation for the ratio of the outlet cross-sectional area to the inlet cross-sectional area (Ao)/(A), it can be .seen that decreasing (W) effects a corresponding decrease in the ratio (Ao)/(Ai) without effecting a corresponding change in the abscissa coordinate of curve A of FIG. 2. This results in curve A shifting away from the ordinate and toward the abscissa so as to intersect the enclosed area of maximum eflciency (80%) on the graph.

Gas flow exists from each merging passage 46 and commingles with the flow exiting from adjacent passages in an annular manifold 60 shown in FIG. 5. The commingling of flow exiting from the transitional passages is accomplished in a smooth and uniform manner with a minimum of turbulence and loss. Flow in the annular manifold 60, however, may include an undesirable swirl component which can be removed by circumferentially spaced apart deswirl vanes 62 which act to cancel the residual circumferential velocity component of the flow exiting from the diffuser ring 30. Conduit 64 may direct the ow emanating from the swirl vanes 62 to a combustion chamber (not shown) as used in a gas turbine engine.

While a preferred embodiment of the present invention has been depicted and described, it will be understood that many modifications and changes may be made thereto without departing from the inventions fundamental theme.

Having thus described one embodiment of the invention, what is desired to be secured by Letters Patent is as follows:

1. A diffuser comprises: an annular housing having a plurality of circumferentially spaced apart passages extending through the annulus wherein the passages initially assume curvilinear inlet cross-sections on the inner peripheral surface of the annulus and gradually merge towards rectilinear outlet cross-sections, on the outer peripheral surface of the annulus with at least one outer circumferential edge of the annulus being bevelled to define a scalloped frusto-conical surface;

and a frusto-conical annulus disposed contiguous with the scalloped frusto-conical surface to define one nominally linear edge of the rectilinear outlet crosssection.

2. A diffuser surrounding a centrifugal compressor which comprises:

an annular housing having a plurality of circumferentially spaced apart passages extending in an outward, generally radial direction through the annulus where the passages initially assume curvilinear inlet cross-sections on the inner peripheral surface of the annulus and gradually merge toward rectilinear outlet cross-sections, on the outer peripheral surface of the annulus, with the outer circumferential edges of the annulus bevelled to define two converging, scalloped frusto-conical surfaces; and at least two frusto-conical annuli, each annuli of which is contiguous with a frusto-conical surface and defines one nominally linear edge of the rectilinear outlet crosssection.

3. The diffuser of claim 2 wherein the rectilinear outlet cross-sections of the passages are defined by axially eX- tending, narrow edges between the passages and the outer circumferential edges of the frusto-conical annuli.

4. The diffuser of claim 3 wherein:

the passages initially assume a circular inlet cross-section and gradually merge into a rectangular outlet cross-section, and the width of each rectangular outlet cross-section coincides with the length of each axially extending edge and is of shorter length than the diameter of each inlet cross-section.

5. The diffuser of claim 2 wherein:

a merging portion of each passage at the outer radial end is initially of curvilinear cross-section and gradually merges into a rectilinear cross-section with the cross-sectional area of the merging portion remaining substantially constant throughout its length, and the remaining initial portion of each passage is of curvilinear cross-section gradually widening in an outward radial direction so as to convert the high kinetic energy with which the owing medium leaves the compressor into static pressure energy.

References Cited UNITED STATES PATENTS 2,596,646 5/1952 Buchi 415-207 2,925,952 2/1960 Garve 415-211 3,289,921 12/1966 Soo 415-207 3,333,762 8/1967 Vrana 415-207 3,658,437 4/1972 Soo 415-211 FOREIGN PATENTS `685,814 1/ 1953 Great Britain 415-207 816,875 7/ 1959 Great Britain 415-207 801,242 12/ 1950 Germany 415-211 17,019 4/ 1903 Sweden 415-211 HENRY F. RADUAZO, Primary Examiner U.S. Cl. X.R. 

